Telecommunications-assisted satellite positioning system

ABSTRACT

A wireless terminal and auxiliary system are disclosed that enable the wireless terminal to determine its location based on signals transmitted from navigation satellites. In particular, the tasks of signal acquisition and signal processing required of a wireless terminal in the prior art are divided between the wireless terminal and the auxiliary system in accordance with the illustrative embodiment. The auxiliary system assists the wireless terminal by acquiring information about the satellites&#39; ephemerides, by partially processing it and by transmitting the partially processed information to the wireless terminal in a form that is useful to the wireless terminal. The wireless terminal then uses the partially processed information from the auxiliary system to assist the wireless terminal in acquiring the ranging signals from the navigation satellites quickly and when they are weak.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.08/927,432, filed Sep. 11, 1997, now pending.

FIELD OF THE INVENTION

The present invention relates to a wireless terminal in general, and,more particularly, to a wireless terminal that receives one or moresignals from an auxiliary system that assists the wireless terminal inreceiving one or more signals from a satellite positioning system.

BACKGROUND OF THE INVENTION

A satellite positioning system, such as the Global Positioning System(“GPS”), comprises a constellation of satellites that transmit signalsthat can be used by a wireless terminal to determine, in well-knownfashion, the wireless terminal's position. Typically, the signalstransmitted by each satellite convey three types of information: (1)satellite trajectory data, (2) system timing, and (3) ranginginformation. When a wireless terminal can acquire the signals from threeor more satellites the wireless terminal can determine its positionthrough triangulation, as is well-known in the art. FIG. 1 depicts aschematic diagram of a satellite positioning system in the prior art.

Although a conventional wireless terminal can determine its positionwith some degree of accuracy, fluctuations in the ionosphere and theatmosphere and jitter in the transmitted signals themselves prevent aconventional wireless terminal from determining its position with a highdegree of accuracy. To mitigate the effects of these factors and thusimprove the degree of accuracy with which a wireless terminal canascertain its position, another satellite positioning system, typifiedby the Differential Global Positioning System (“DGPS”), was developed.FIG. 2 depicts a schematic diagram of a Differential Global PositioningSystem.

As is well-known in the prior art, DGPS comprises terrestrial referencereceiver 205, whose position is static and exactly known throughconventional survey techniques, in addition to satellite constellation203 and wireless terminal 201. The theory underlying DGPS is that whenwireless terminal 201 is in close proximity (e.g., within 50 miles) toterrestrial reference receiver 205, both wireless terminal 201 andterrestrial reference receiver 205 are expected to experience the sameionospheric and atmospheric fluctuations and signal jitter. Terrestrialreference receiver 205 uses the signals from satellite constellation 203to estimate its position, and, using its known exact position,calculates the error between its estimated position and its known exactposition. That error or “difference” is a vector that represents theinaccuracy of the estimated position from the ionospheric andatmospheric fluctuations and signal jitter. The difference vector isbroadcast by terrestrial reference receiver 205 to wireless terminal 201in real time. When wireless terminal 201 estimates is position throughconventional means, it uses the difference vector received fromterrestrial reference receiver 205 to subtract out the effects of theionospheric and atmospheric fluctuations and signal jitter.

FIG. 3 depicts a schematic diagram of a Tidget® satellite positioningsystem in the prior art. The wireless receiver in a Tidget system doesnot compute the position of the wireless terminal. Instead, the wirelessreceiver in a Tidget system acts like a wireless repeater in that itreceives the signals from the satellite constellation and then relaysthe unprocessed signals to a remote processing facility, which uses thesignals to determine the position of the Tidget wireless terminal. Anadvantage of a Tidget system is that is reduces the cost of the wirelessterminal by eliminating from the wireless terminal the expensivecircuitry that would otherwise be needed to compute the position of thewireless terminal. When it is more advantageous that a remote facilityknow the location of the wireless terminal than that the wirelessterminal know its own location, a Tidget system is advantageous in thatit relays, in effect, the position of the wireless terminal to theremote facility.

FIG. 4 depicts a schematic diagram of a Tendler® satellite positioningsystem in the prior art. A wireless terminal constructed in accordancewith this system comprises both the circuitry needed to determine itsposition from a satellite constellation and a wireless telephonetransmitter to transmit the determined position to another party via awireless telecommunications system.

Regardless of the advances made in satellite positioning systems,limitations still exist. Typically, the strength of the signals from thesatellite constellation is too attenuated in buildings and othershadowed environments for a wireless terminal to receive. Furthermore, awireless terminal can take several minutes to acquire the signals fromthe satellites it needs to determine its position.

SUMMARY OF THE INVENTION

Some embodiments of the present invention are capable of determining theposition of a wireless terminal while avoiding many of the costs andrestrictions associated with systems in the prior art. In particular,some embodiments of the present invention are less expensive thanwireless terminals in the prior art. Furthermore, some embodiments ofthe present invention are able to receive and use weaker signals thanwireless terminals in the prior art; and still furthermore, someembodiments of the present invention are capable of determining theirlocation more quickly than wireless terminals in the prior art.

A wireless terminal in accordance with an embodiment of the presentinvention can exhibit these advantages when the tasks of signalacquisition and signal processing required of a wireless terminal in theprior art are divided between the wireless terminal and an auxiliarysystem. In particular, the requirements normally imposed on a wirelessterminal in the prior art are off-loaded onto an auxiliary system thatcan provide useful information to the wireless terminal over a wirelesstelecommunications link.

It is possible to divide the signal acquisition and signal processingtasks between the wireless terminal and the auxiliary system becauseeach signal transmitted by each satellite in a satellite positioningsystem's constellation carries two distinct kinds of information thatare responsive to independent acquisition and independent processing.The two kinds of information are: (1) ranging information, and (2)information about the satellites' ephemerides.

The information about the satellites' ephemerides is the same for allreceivers, regardless of their position. In contrast, the ranginginformation, which indicates to the receiver its distance from eachsatellite, is location dependent and can be received only by thewireless terminal itself. Therefore, the auxiliary system can assist thewireless terminal by acquiring the information about the satellites'ephemerides, by partially processing it and by transmitting it to thewireless terminal in a form that is useful to the wireless terminal. Theauxiliary system cannot, however, acquire the ranging information forthe wireless terminal.

By having the auxiliary system acquire the information about thesatellites' ephemerides for the wireless terminal, the signalacquisition and signal processing demands of the wireless terminal arereduced. Furthermore, the wireless terminal can actually use thepartially processed information from the auxiliary system to assists thewireless terminal in acquiring the ranging signals quickly and when theyare weak.

When the wireless terminal is capable of providing the functionality ofa wireless telecommunications terminal (e.g., a cellular telephone, ahand-held data entry device, etc.), the circuitry for determining thewireless terminal's location, in accordance with some embodiments of thepresent invention, can be added to the wireless terminal for moderatelylittle cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a satellite positioning system, such asGPS, in the prior art.

FIG. 2 is a block diagram of a differential GPS system in the prior art.

FIG. 3 is a block diagram a Tidget-like system in the prior art.

FIG. 4 is a block diagram of a Tendler-like system in the prior art.

FIG. 5 is a block diagram of a satellite positioning system inaccordance with the illustrative embodiment of the present invention.

FIG. 6 is a block diagram of the auxiliary system shown in FIG. 5.

FIG. 7 is a block diagram of the wireless terminal shown in FIG. 5.

FIG. 8 is a block diagram of the field receiver shown in FIG. 7.

FIG. 9 is a flowchart of the operation of the auxiliary system andwireless terminal shown in FIG. 5 in accordance with one embodiment ofthe present invention.

FIG. 10 is a flowchart of the operation of the auxiliary system andwireless terminal shown in FIG. 5 in accordance with another embodimentof the present invention.

DETAILED DESCRIPTION

FIG. 5 depicts a drawing of a satellite positioning system in accordancewith an illustrative embodiment of the present invention. The satellitepositioning system depicted comprises wireless terminal 501, satelliteconstellation 503, auxiliary system 505 and timing source 507. Satelliteconstellation 503 is the Global Positioning System as is well-known inthe art and will not be further discussed. It will be clear to thoseskilled in the art how to make and use embodiments of the presentinvention that work with other satellite constellations.

The principal goal of the illustrative embodiment is to reduce thesignal acquisition and signal processing requirements of a conventionalwireless terminal so that a wireless terminal in accordance with theillustrative embodiment can determine its location more quickly and withweaker signals than wireless terminals in the prior art. In accordancewith the illustrative embodiment, the signal acquisition and signalprocessing requirements of wireless terminal 501 are reduced at theexpense of auxiliary system 505. In particular, the tasks of signalacquisition and signal processing required for a conventional wirelessterminal to determine its position are divided between wireless terminal501 and auxiliary system 505.

It will be clear to those skilled in the art how the signal processingtask can be divided between wireless terminal 501 and auxiliary system505, as partially processed signal information can be exchanged back andforth between the two through wireless telecommunications link 504 asneeded to achieve desirable division of the signal processing task.

It is possible to divide the signal processing task between wirelessterminal 501 and auxiliary system 505 because each signal transmitted byeach satellite in satellite constellation 503 carries two distinct kindsof information that are responsive to independent acquisition andindependent processing. The two kinds of information are: (1) ranginginformation, and (2) information about the satellites' ephemerides. Morespecifically, the GPS signal is modulated with digital information in amanner similar to how, for example, a cellular telephone's radio signalis modulated with voice data. Such information can be detected anddemodulated by any receiver adapted to do so. The informationreconstructed by the receiver is an exact replica of the informationmodulated onto the signal by the transmitter (except for unwanted errorsdue to noise, distortion, etc.) and is the same for all receivers,regardless of their position. This information shall be referred to as“information about the satellites' ephemerides.”

In contrast, in a location system there is also important information inthe precise timing of the signal. The transmitter carefully adjusts thetiming of the transmitted signal according to some precise reference,such that the timing of the signal, as received by the receiver carriesinformation about the distance between the transmitter and the receiver(and, therefore, about the receiver's position). Such information willbe different from receiver to receiver, and is only available at thereceiver itself This information shall be referred to as “ranginginformation.”

For example, since each satellite in constellation 503 transmits asignal 502 that contains both kinds of information to both wirelessterminal 501 and auxiliary system 505, some or all of the informationabout the satellites' ephemerides is acquired by auxiliary system 505through antenna 553, even though the ranging information acquired byauxiliary system 505 is relevant to the position of auxiliary systemantenna 553 and not to the position of wireless terminal 501. However,auxiliary system 505 has approximate knowledge of the position ofwireless terminal 501 (for example, through knowledge of the cell andsector where a mobile is located); therefore, auxiliary system 505combines this knowledge with the acquired ranging information and withthe satellites' ephemerides information to compute an estimate of theranging information at the position of wireless terminal 501. Thisestimate, together with the satellites' emphemerides information, istransmitted, via wireless telecommunications antenna 551, to wirelessterminal 501 to assist wireless terminal 501 in acquiring and processingranging information.

Once the ranging information has been acquired by wireless terminal 501,wireless terminal 501 can use the satellite ephemeris information andranging information to determine its location, or wireless terminal 501can transmit the ranging information back to auxiliary system 505 sothat auxiliary system 505 can determine the location of wirelessterminal 501.

Because wireless terminal 501 is freed from the task of acquiring someor all of the information about the satellites' ephemerides and isadvantageously provided with an estimate of the ranging information, itcan be fabricated from less expensive technology that need only performthe easier task of acquiring and processing the ranging information witha priori knowledge of an estimated form of that information.Furthermore, because the satellite ephemerides information is modulatedonto the same carrier as the ranging information, the provision of thesatellites' ephemerides information to wireless terminal 501 enableswireless terminal 501 to remove the satellites' ephemerides informationfrom the satellite signal received through antenna 512 and, thereby,acquire the ranging information even under faded conditions of lowsignal-to-noise ratio that are inadequate for the operation of awireless terminal in prior art.

Auxiliary system 505 can be a terrestrial facility, an airborne facilityor an artificial satellite in orbit around the earth. Unlike aDifferential Global Positioning System's terrestrial reference receiver,however, the position of auxiliary system 505 need not remain static norneed its exact location be known.

FIG. 6 depicts a block diagram of the salient components of auxiliarysystem 505, which comprises: timing signal receiver 603, timing signalantenna 552, coarse location estimator 601, telecommunications systemmanager 617, GPS receiver 605, GPS receiver antenna 553, timing signalcalibrator 607, PRN synchronization estimator 609, demodulator 611,satellite visibility estimator 613, satellite Doppler estimator 615,telecommunications transmitter 619 and telecommunications antenna 551.

In general, auxiliary system 505 uses its GPS receiver to obtain fromeach satellite above the horizon both ranging information andinformation about the satellite's ephemeris, in well-known fashion usingthe C/A or Coarse Acquisition code. It will be clear to those skilled inthe art how to make and use embodiments of the present invention thatuse the P(Y) or P code. In the process of obtaining the ranging andsatellite ephemeris information, auxiliary system 505 learns, amongother things: (1) the pseudo-random number (hereinafter “PRN”)synchronization from each satellite (i.e., the exact timing of the PRNcode transmitted by each satellite), (2) the Doppler shift associatedwith each satellite, (3) which satellites are above the horizon, and (4)the 50 bps modulated bit stream from each satellite. Auxiliary system505 then transmits to wireless terminal 501 , via a wirelesstelecommunications channel, for each satellite above the horizon: (1) anestimate of the PRN synchronization, (2) an estimate of the Dopplershift, and (3) the 50 bps modulated bit stream. Collectively, thisinformation will be called “Navigation Message Data.”

When auxiliary system 505 is part of a wireless telecommunicationssystem that partitions a geographic area into a number of tessellatedareas called “cells,” auxiliary system 505 knows which cell wirelessterminal 501 is in and, therefore, its rough location to within a fewmiles. When auxiliary system 505 has a rough idea (e.g., within a fewmiles) of the position of wireless terminal 501, auxiliary system 505can accurately estimate the PRN synchronization and Doppler shift asseen by wireless terminal 501.

Because the PRN synchronization estimate, the Doppler shift estimate andthe 50 bps modulated bit stream are perishable and only useful whenwireless terminal 501 and auxiliary system 505 are synchronized within afew GPS C/A code chips, both wireless terminal 501 and auxiliary system505 are advantageously synchronized to within 1 μs. To accomplish this,both wireless terminal 501 and auxiliary system 505 can receive a timingsynchronization signal from independent timing source 507, in well-knownfashion. Alternatively, auxiliary system 505 can contain a timing sourceand can transmit a synchronization signal to wireless terminal 501 overthe telecommunications channel.

For example, when auxiliary system 505 is part of a CDMA wirelesstelecommunications system and wireless terminal 501 is CDMA compliant,both auxiliary system 505 and wireless terminal 501 will be synchronizedto within 1 μs and timing source 507 is not needed. It will be clear tothose skilled in the art how to provide synchronization for wirelessterminal 501 and auxiliary system 505.

Returning to FIG. 6, when auxiliary system 505 is part of an IS-95 CDMAtelecommunications system, telecommunications system manager 617 informscoarse location estimator 601 of the cell in which wireless terminal 501is located. Furthermore, telecommunications system manager 617 caninstigate the process of locating wireless terminal 501 when, forexample, wireless terminal 501 is carried by a lost child. As anotherexample, a “911” emergency-services call from wireless terminal 501 canprovoke telecommunications system manager 617 to locate wirelessterminal 501 and direct emergency service personnel to the location ofwireless terminal 501. Another position-based service could enable aperson whose car had broken down to enter a code, such as *TOW, intowireless terminal 501. Wireless terminal 501 would then relay *TOW totelecommunications system manager 617, which would then ascertain theposition of wireless terminal 501 and establish a call between wirelessterminal 501 and the towing service that was closest to wirelessterminal 501. The disclosure of pending U.S. patent application Ser. No.08/784108, filed Jan. 15, 1997, entitled “Wireless Location Messaging,”is incorporated by reference.

Coarse location estimator 601 uses the information fromtelecommunications system manager 617 to produce an estimate of thelatitude and longitude of the location of wireless terminal 501, whichestimate could simply be the location of the center of the cell orsector containing wireless terminal 501.

Timing signal receiver 603 receives the same timing signal from timingsource 507 that is received by wireless terminal 501, when timing source507 is needed for synchronization. The locations of timing signalreceiver 603 and timing source 507 must be known with sufficientaccuracy to allow timing signal calibrator 607 to accurately determinethe timing signal delay between timing source 507 and timing signalreceiver 603, as well as the timing signal delay between timing source507 and wireless terminal 501. For example, the required timing accuracycould be 1 μsec, based on the coarse estimate of the location ofwireless terminal 501. Alternatively, timing signal receiver 603 couldreceive the timing signal from GPS constellation 503.

GPS receiver 605 receives a signal, via GPS receiver antenna 553, fromeach satellite in satellite constellation 503 above the horizon anddetermines each signal's exact time of arrival (i.e., its PRNsynchronization). Demodulator 611 demodulates each acquired signal torecover its 50 bps modulated bit stream. PRN synchronization estimator609 predicts the exact time of arrival of each C/A code signal from eachvisible satellite at wireless terminal 501 and uses these predictions toestimate the PRN sequence timing to be used by the field receiver inwireless terminal 501 for proper de-spreading of the respective C/A codesignals. It should be understood that although PRN synchronizationestimator 609 cannot determine the exact PRN sequence timing at wirelessterminal 501, a good estimate (e.g., one that is correct within 10 or 20chips) substantially reduces the number of trial PRN synchronizationsthat wireless terminal 501 would otherwise have to try.

Satellite visibility estimator 613 extracts the satellite ephemeris fromthe received modulation bit streams and estimates which satellites arevisible to wireless terminal 501 at its location. Similarly, satelliteDoppler estimator 615 extracts satellite ephemeris information from thereceived modulation bit streams and estimates which satellites arevisible to wireless terminal 501 at its location. Telecommunicationstransmitter 619 takes the satellite visibility estimate, the PRNsynchronization estimate for each satellite, the Doppler shift estimatefor each satellite and the 50 bps modulated bitstream for each satelliteand transmits to wireless terminal 501 over a telecommunications channelfor each satellite above the horizon: (1) an estimate of the PRNsynchronization, (2) an estimate of the Doppler shift, and (3) the 50bps modulated bit stream. It will be clear to those skilled in the arthow to make and use auxiliary system 505.

FIG. 7 depicts a block diagram of the major components of wirelessterminal 501, which comprises: terminal controller 710, user interface720, telecommunications transmitter 741, telecommunications receiver751, field receiver 753, timing receiver 755, duplexor 733 and antenna731, interconnected as shown.

Advantageously, but not necessarily, wireless terminal 501 is capable ofperforming all of the functionality associated with a typical wirelessterminal (e.g., a cellular telephone,). In particular a user of wirelessterminal is advantageously capable of having a two-way voiceconversation through telecommunications transmitter 741,telecommunications receiver 751 and auxiliary system 505.

Because the Navigation Message Data is transmitted to wireless terminal501 from auxiliary system 505, the Navigation Message Data is receivedby wireless terminal 501 via telecommunications receiver 75 1.Telecommunications receiver 751 passes the Navigation Message Data toterminal controller 710, which, in turn, passes the Navigation MessageData to field receiver 753.

As discussed above, wireless terminal 501 also advantageously receivessystem timing for synchronization purposes. When the timing signal istransmitted from timing source 507, the timing signal is received bywireless terminal 501 via timing receiver 755. Timing receiver 755passes the timing signal to terminal controller 710 which, in turn,passes the timing signal to field receiver 753. Alternatively, when thetiming signal is transmitted from auxiliary system 505, (as is the casewhen wireless terminal 501 and auxiliary system 505 are part of a CDMAtelecommunications system) the timing signal is received bytelecommunications receiver 741. Telecommunications receiver 741 thenpasses the timing signal to terminal controller 710 which, in turn,passes the timing signal to field receiver 753.

In either case, field receiver 753 receives the timing information thatit needs without needing to derive it from satellite constellation 503.Furthermore, field receiver 753 also receives for each satellite abovethe horizon: (1) an estimate of the PRN synchronization, (2) an estimateof the Doppler shift, and (3) the 50 bps modulated bit stream, againwithout having received any of this information directly from satelliteconstellation 503.

Wireless terminal 501 also advantageously receives the direct sequencespread spectrum C/A code signals from satellite constellation 503 viafield receiver 753.

FIG. 8 depicts a block diagram of the major components of field receiver753 that process the C/A code signal from one satellite in satelliteconstellation 503. For pedagogical reasons, the functions of fieldreceiver 753 are depicted in FIG. 8 as separate functional blocks thatoperate on one C/A code signal. It will be clear to those skilled in theart that in many embodiments of the present invention field receiver 753will be an appropriately programmed general-purpose microprocessor ordigital signal processor that simultaneously operates on C/A codesignals from multiple satellites. It will also be clear to those skilledin the art that many of the functional blocks in FIG. 8 can besubstituted for by transform techniques.

In FIG. 8, SPS controller 821 advantageously receives the NavigationMessage Data and timing synchronization information from lead 761 andoutputs: (1) the PRN synchronization estimate to PRN code generator 819,(2) the Doppler shift estimate to Doppler correction 809, and the 50 bpsmodulation bit stream to mixer 815 and location computer 823, allappropriately synchronized. RF front end 801 receives the C/A codesignal from a satellite, filters out everything other than the band ofinterest and mixes it down to IF in well known fashion. A/D converter803 takes the mixed-down signal and samples it at twice the chippingrate of 1.023 MChips/sec. or more. PRN code generator 819 beginsgenerating the PRN code sequence at 1.023 MChips/sec., which PRN codesequence has a period of 1023 chips, as is well-known in the art. PRNcode generator 819 can also use the Doppler shift estimate to correctthe PRN code sequence chip rate for Doppler shift, but, because theDoppler shift on the PRN code sequence is usually very small, this neednot always be done. It will be clear to those skilled in the art whenPRN code generator 819 can neglect correcting for Doppler shift and whenit can not.

It will be understood by those skilled in the art how the signalprocessing functions performed by the blocks that follow A/D converter803 in FIG. 8 can also be performed in alternative embodiments usinganalog techniques. In such embodiments, field receiver 753 will bedescribed by a block diagram similar to the one of FIG. 8 except thatA/D converter 803 will appear at a different point in the functionalsequence of blocks.

It should be understood that no guarantee is needed that the PRNsynchronization estimate be correct or that the first PRN code sequencefrom PRN code generator 819 be synchronized exactly. If it turns outthat the PRN code sequence from PRN code generator 819 is notsynchronized (as is determined by spectral analyzer 817), the PRN codegenerator 819 will use the PRN synchronization estimate as an educatedguess at finding the true synchronization through a progressive searchof synchronization positions near the estimate, in well-known fashion.

Mixer 805 multiplies the PRN code sequence and the digitized C/A codesignal and outputs the despread C/A code to lowpass filter 807. Lowpassfilter 807 advantageously reduces the bandwidth of the signal so that itcan be sampled at a lower rate. This allows Doppler correction block 809to ignore all but one out of every several samples it receives fromlowpass filter 807, so that the resulting number of samples per secondis at least the Nyquist rate needed for accurate representation of theoutput of lowpass filter 807, or twice the bandwidth occupied by theoutput of lowpass filter 807. Advantageously, the bandwidth is equal tothe largest Doppler shift observable in the signal (caused by therelative motion of the satellite with respect to wireless terminal 501)increased by the bandwidth occupied by the 50-bps signal itself. Forexample, the bandwidth occupied by the output of Lowpass filter 807 canbe 8 kHz, corresponding to a Nyquist rate of 16 kilosamples/s).

The Doppler shift caused by the relative motion of the satellite withrespect to wireless terminal 501 is comprised of two components: aDoppler shift caused by the relative motion of the satellite withrespect to ground (for which an estimate is included in the navigationmessage data) and a Doppler shift caused by the relative motion, if any,of wireless terminal 501 with respect to ground. Doppler correction 809takes the signal from lowpass filter 807 and corrects for the estimatedDoppler shift due to the relative motion of the satellite with respectto ground. This can be accomplished, in well-known fashion, through, forexample, frequency conversion techniques where the frequency of a localoscillator is adjusted to achieve the desired correction.

The output of Doppler correction 809 is fed into lowpass filter 811which advantageously further reduces the bandwidth of the signal so thatit can be sampled at a yet lower rate. Again, FIFO 813 can ignore allbut one out of every several samples it receives from lowpass filter811. The samples that are not ignored must occur at a rate that is atleast a Nyquist rate equal to twice the bandwidth occupied by the outputof lowpass filter 811. Advantageously, the bandwidth is equal to thelargest Doppler shift caused by the relative motion of wireless terminal501 with respect to ground increased by the bandwidth occupied by the50-bps signal itself. For example, the bandwidth occupied by the outputof lowpass filter 811 can be 500 Hz, corresponding to a Nyquist rate of1 kilosamples/s).

The output of lowpass filter 811 is fed into FIFO memory 813, whichdelays the signal for only so long as it takes auxiliary system 505 torecover the 50 bps modulated bit stream and forward it to SPS controller821. Typically, FIFO memory 813 need only delay the signal for, at most,a few seconds. The output of FIFO memory 813 is fed into mixer 815 to bemixed with the carefully synchronized 50 bps modulated bit stream. Themixing operation will further de-spread the signal by removing the50-bps modulation. As a result, the output of mixer 815 will be theunmodulated signal carrier, if a signal is present (i.e., if the PRNsynchronization is correct).

The output of mixer 813 is fed into spectral analyzer 817, whichperforms, for example, a discrete fourier transform in well-knownfashion. When the output of mixer 813 is a pure sinusoid (which isindicated by a spectral spike out of spectral analyzer 817), it meansthat PRN code generator 819 is perfectly in sync with the C/A codesignal from the satellite. When the output of mixer 813 is other than apure sinusoid (which is indicated by something other than a spectralspike out of spectral analyzer 817), it means that PRN code generator819 is not in sync with the C/A code signal and must try anothersynchronization. It will be clear to those skilled in the art how toperform the spectral analysis through techniques different than thosedescribed here, however, that yield the same result, which is detectingthe presence or absence of a narrowband component in the output of mixer815.

Importantly, when PRN code generator is in sync with the C/A code signalfrom the satellite, it means that location computer 823 can compute theranging information (i.e., how long did it take the signal to travelfrom the satellite to wireless terminal 501). And because locationcomputer 823 knows: (1) the PRN code synchronization from PRN codegenerator 819, (2) the modulated bit stream from SPS controller 821 and(3) when the PRN code is synchronized from spectral analyzer 817,location computer 823 can compute the location of wireless terminal 501,in well-known fashion.

The location of wireless terminal 501 can then be output from locationcomputer 823 to terminal controller 710 and to telecommunicationstransmitter 741 for transmission back to auxiliary system 505 over atelecommunications channel. Auxiliary system 505 can then use thelocation of wireless terminal 501 in any number of location-basedservices.

FIG. 9 is a flowchart of the operation of the auxiliary system andwireless terminal shown in FIG. 5 in accordance with one embodiment ofthe present invention.

FIG. 10 is a flowchart of the operation of the auxiliary system andwireless terminal shown in FIG. 5 in accordance with another embodimentof the present invention.

What is claimed is:
 1. A wireless terminal comprising: atelecommunications receiver for receiving a code synchronizationestimate for a ranging signal from a satellite from an auxiliary systemover a wireless telecommunications link; and a field receiver forreceiving and processing said ranging signal using said codesynchronization estimate.
 2. The wireless terminal of claim 1 whereinsaid telecommunications receiver also receives a Doppler shift estimatefor said ranging signal from said auxiliary system, and wherein saidfield receiver uses said Doppler shift estimate in processing saidranging signal.
 3. The wireless terminal of claim 1 wherein saidtelecommunications receiver also receives a modulation bit sequence forsaid ranging signal from said auxiliary system, and wherein said fieldreceiver uses said modulation bit sequence in processing said rangingsignal.
 4. The wireless terminal of claim 1 wherein saidtelecommunications receiver also receives a system timing signal fromsaid auxiliary system, and wherein said field receiver uses said systemtiming signal in processing said ranging signal.
 5. The wirelessterminal of claim 1 wherein said telecommunications receiver alsoreceives a system timing signal from an independent timing source, andwherein said field receiver uses said system timing signal in processingsaid ranging signal.
 6. The wireless terminal of claim 1 wherein saidwireless terminal uses said code synchronization estimate and saidranging signal to determine a position of said wireless terminal.
 7. Thewireless terminal of claim 6 further comprising a telecommunicationstransmitter for transmitting said position of said wireless terminal tosaid auxiliary system.
 8. The wireless terminal of claim 6 furthercomprising a visual display for conveying said position of said wirelessterminal to a user of said wireless terminal.
 9. The wireless terminalof claim 1 wherein said wireless terminal creates a partially processedranging signal based on said synchronization estimate and said rangingsignal; and further comprising a telecommunications transmitter fortransmitting said partially processed ranging signal to said auxiliarysystem.
 10. The wireless terminal of claim 1 wherein said wirelessterminal creates a ranging estimate based on said synchronizationestimate and said ranging signal; and further comprising atelecommunications transmitter for transmitting said ranging estimate tosaid auxiliary system.
 11. The wireless terminal of claim 1 wherein saidfield receiver uses: said code synchronization estimate to facilitatethe creation of a candidate code; said candidate code and said rangingsignal to create a candidate sinusoidal signal; and further comprising adiscrete fourier transform of said candidate sinusoidal signal fordetermining when said candidate code is synchronized with said rangingsignal.
 12. A wireless terminal comprising: a telecommunicationsreceiver for receiving a modulation bit sequence for a ranging signalfrom a satellite from an auxiliary system over a wirelesstelecommunications link; and a field receiver for receiving andprocessing said ranging signal using said modulation bit sequence. 13.The wireless terminal of claim 12 wherein said telecommunicationsreceiver also receives a Doppler shift estimate for said ranging signalfrom said auxiliary system, and wherein said field receiver uses saidDoppler shift estimate in processing said ranging signal.
 14. Thewireless terminal of claim 12 wherein said telecommunications receiveralso receives a code synchronization estimate for said ranging signalfrom said auxiliary system, and wherein said field receiver uses saidcode synchronization estimate in processing said ranging signal.
 15. Thewireless terminal of claim 12 wherein said telecommunications receiveralso receives a system timing signal from said auxiliary system, andwherein said field receiver uses said system timing signal in processingsaid ranging signal.
 16. The wireless terminal of claim 12 wherein saidtelecommunications receiver also receives a system timing signal from anindependent timing source, and wherein said field receiver uses saidsystem timing signal in processing said ranging signal.
 17. The wirelessterminal of claim 12 wherein said wireless terminal uses said modulationbit sequence and said ranging signal to determine a position of saidwireless terminal.
 18. The wireless terminal of claim 17 furthercomprising a telecommunications transmitter for transmitting saidposition of said wireless terminal to said auxiliary system.
 19. Thewireless terminal of claim 17 further comprising a visual display forconveying said position of said wireless terminal to a user of saidwireless terminal.
 20. The wireless terminal of claim 12 wherein saidwireless terminal creates a partially processed ranging signal based onsaid modulation bit sequence and said ranging signal; and furthercomprising a telecommunications transmitter for transmitting saidpartially processed ranging signal to said auxiliary system.
 21. Thewireless terminal of claim 12 wherein said wireless terminal creates aranging estimate based on said modulation bit sequence and said rangingsignal; and further comprising a telecommunications transmitter fortransmitting said ranging estimate to said auxiliary system.
 22. Thewireless terminal of claim 12 wherein said field receiver uses: acandidate code, said modulation bit sequence, and said ranging signal tocreate a candidate sinusoidal signal; and further comprising a discretefourier transform of said candidate sinusoidal signal for determiningwhen said candidate code is synchronized with said ranging signal.
 23. Amethod of operating a wireless terminal, said method comprising:receiving a ranging signal from a satellite; receiving a codesynchronization estimate for said ranging signal from an auxiliarysystem; and processing said ranging signal using said codesynchronization estimate.
 24. The method of claim 23 further comprising:receiving a modulation bit sequence for said ranging signal from saidauxiliary system; and processing said ranging signal using saidmodulation bit sequence.
 25. The method of claim 23 further comprising:receiving a Doppler shift estimate for said ranging signal from saidauxiliary system; and processing said ranging signal using said Dopplershift estimate.
 26. The method of claim 23 further comprising: receivingfrom a system timing signal; and processing said ranging signal usingsaid system timing signal.
 27. The method of claim 26 wherein saidsystem timing signal is received from said auxiliary system.
 28. Themethod of claim 26 wherein said system timing signal is received from anindependent timing source.
 29. The method of claim 23 further comprisingdetermining a position of said wireless terminal based on said codesynchronization estimate and said ranging signal.
 30. The method ofclaim 29 further comprising transmitting said position of said wirelessterminal to said auxiliary system.
 31. The method of claim 29 furthercomprising visually displaying a position of said wireless terminal to auser of said wireless terminal.
 32. The method of claim 23 furthercomprising: creating a partially processed ranging signal based on saidcode synchronization estimate and said ranging signal; and transmittingsaid partially processed ranging signal to said auxiliary system. 33.The method of claim 23 further comprising: creating a ranging estimatebased on said code synchronization estimate and said ranging signal; andtransmitting said ranging estimate to said auxiliary system.
 34. Themethod of claim 23 further comprising: creating a candidate code basedon said code synchronization estimate; creating a candidate sinusoidalsignal based on said ranging signal and said candidate code; andperforming spectral analysis on said candidate sinusoidal signal todetermine when said candidate code is synchronized with said rangingsignal.
 35. A method of operating a wireless terminal, said methodcomprising: receiving a ranging signal from a satellite; receiving amodulation bit sequence for said ranging signal from said auxiliarysystem; and processing said ranging signal using said modulation bitsequence.
 36. The method of claim 35 further comprising: receiving acode synchronization estimate for said ranging signal from an auxiliarysystem; and processing said ranging signal using said codesynchronization estimate.
 37. The method of claim 35 further comprising:receiving a Doppler shift estimate for said ranging signal from saidauxiliary system; and processing said ranging signal using said Dopplershift estimate.
 38. The method of claim 35 further comprising: receivingfrom a system timing signal; and processing said ranging signal usingsaid system timing signal.
 39. The method of claim 38 wherein saidsystem timing signal is received from said auxiliary system.
 40. Themethod of claim 38 wherein said system timing signal is received from anindependent timing source.
 41. The method of claim 35 further comprisingdetermining a position of said wireless terminal based on saidmodulation bit sequence and said ranging signal.
 42. The method of claim41 further comprising transmitting said position of said wirelessterminal to said auxiliary system.
 43. The method of claim 41 furthercomprising visually displaying a position of said wireless terminal to auser of said wireless terminal.
 44. The method of claim 35 furthercomprising: creating a partially processed ranging signal based on saidmodulation bit sequence and said ranging signal; and transmitting saidpartially processed ranging signal to said auxiliary system.
 45. Themethod of claim 35 further comprising: creating a ranging estimate basedon said modulation bit sequence and said ranging signal; andtransmitting said ranging estimate to said auxiliary system.
 46. Themethod of claim 35 further comprising: creating a candidate sinusoidalsignal based on said ranging signal and a candidate code; and performingspectral analysis on said candidate sinusoidal signal to determine whensaid candidate code is synchronized with said ranging signal.